Hi nick, maybe I don't get exactly what you need. By the way, if you need more information about the polynomial interpolation you can find out more looking for 'Lagrange interpolation' on any algebra book or at http://en.wikipedia.org/wiki/Polynomial_interpolation . If you need a tool to work with symbolic calculation, I suggest you maxima and (http://maxima.sourceforge.net/) the symbolic matlab toolbox.

In your previous comment, you speak about "Lagrange interpolation" and I remember using this method on a series to get "intermediate" values. I used scipy.interpolate.lagrange for this but this function needs to be given an extract of the series. Indeed, the length of its parameters gives the degree of the polynomial (minus 1 I guess).

What does polyfit compared to interpolate.lagrange ? Does it select the best points to create what I call the "sub series" ? Thanks for your answer :)